Portable electronic devices (e.g., notebook computers, cellular telephone, cordless telephones mobile data terminals, radio frequency portable communication devices, etc.) typically include a rechargeable battery that is charged by a recharging unit plugged into an AC power source, such as that found in conventional 115 VAC lines. Certain rechargeable batteries can be recharged by coupling the rechargeable battery to a DC voltage source (e.g., car adapter, plane adapter, airplane adapter, USB power bus) The recharging unit powers the portable device, while simultaneously charging the rechargeable battery. The portable device switches over to battery power upon removal of the portable device from the charging unit or the power source. Some portable electronic devices are provided with two or more batteries, so that the portable electronic device can be used for longer periods of time than is possible with a single battery.
In systems with multiple batteries, a switching network is required to allow selective charging and discharging of each battery pack, while maintaining isolation between battery packs, a battery charger and a load. In normal operation, each battery can be in a different state of charge and, as a result, have different output voltages. This characteristic makes the use of single switches not practical, as the parasitic back-gate diode, associated with discrete power Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) when in an off mode, will cause the system to go in a diode-OR configuration. In general this is unacceptable, as most systems require each battery to be isolated from the others during charging and discharging.
Conventional solutions utilize two MOSFETs in a back-to-back configuration to isolate the batteries, the charger and the load. The back-to-back configuration provides isolation by assuring that one or the other of the body diodes is reverse-biased at all times. The switching of the MOSFETs can be controlled by logic signals from a control system. Conventional two battery systems have existing switching network configurations that use a maximum of eight back-to-back MOSFETS and four control signals or six back-to-back MOSFETS and six control signals. FIG. 1 illustrates an example of a switching network having an eight MOSFET configuration, while FIG. 2 illustrates a switching network having a six MOSFET configuration.
FIG. 1 illustrates a multiple battery system 10 with a switching network having an eight MOSFET configuration in accordance with a conventional system. The multiple battery system 10 includes a first switching cell 20 having a first p-type MOSFET Q1 coupled to a second p-type MOSFET Q2 in a back-to-back configuration to isolate a first battery 28 from a load 18. The drains of the first p-type MOSFET Q1 and the second p-type MOSFET Q2 are coupled to one another, such that the back-gate diodes or parasitic diodes of the first p-type MOSFET Q1 and the second p-type MOSFET Q2 have conduction paths in opposite directions. The body diodes formed between the substrate and source (not shown) have opposite conduction paths of the first p-type MOSFET Q1 and the second p-type MOSFET Q2 since the body diodes are in series with the back gate diode and have a conduction direction opposite to the back gate diode. The first switching cell 20 is turned on and off by a single control line /BL1. A second switching cell 22 includes a third p-type MOSFET Q3 coupled to a fourth p-type MOSFET Q4 to isolate the first battery 28 from a charger 16. The second switching cell 22 is configured the same as the first switching cell 20 in a back-to back configuration. The second switching cell 22 is turned on and off by a single control line /B1C.
A third switching cell 24 includes a fifth p-type MOSFET Q5 coupled to a sixth p-type MOSFET Q6 to isolate a second battery 30 from the charger 16. The third switching cell 24 is configured the same as the first and second switching cells 20 and 22, respectively, in a back-to back configuration. The third switching cell 24 is turned on and off by a single control line /B2C. A fourth switching cell 26 includes a seventh p-type MOSFET Q7 coupled to an eighth p-type MOSFET Q8 to isolate the second battery 30 from the load 18. The fourth switching cell 26 is configured the same as the first, the second and the third switching cells 20, 22 and 24, respectively, in a back-to back configuration. The fourth switching cell 26 is turned on and off by a single control line /B2L. A DC supply 12 is coupled to the load 18 through a diode D1. The DC supply 12 is also coupled to the charger 16. When power from the DC supply 12 is available, the DC supply 12 powers the load 18 and the charger 16. The charger 16 can provide charge to either the first battery 28 or the second battery 30 by selecting the second or third switching cells 22 and 24, respectively. When power from the DC supply 12 is not available, the first battery 28 or the second battery 30 can provide power to the load 18 by selecting the first or the fourth switching cells 20 and 26, respectively.
FIG. 2 illustrates a multiple battery system 40 and switching network having a six MOSFET configuration in accordance with a conventional system. The multiple battery system 40 includes a first switching cell 44 having a first p-type MOSFET Q9 coupled to a second p-type MOSFET Q10, a second switching cell 46 having a third p-type MOSFET Q11 coupled to a fourth p-type MOSFET Q12, and a third switching cell 48 having a fifth MOSFET Q13 coupled to a sixth p-type MOSFET Q14. A load 56 is coupled to a node 58 connecting the sources of the third and fourth p-type MOSFETS Q11 and Q12, respectively. A DC supply 54 is also coupled to the node 58 through a diode D2. A first battery 50 is coupled to the drain of the second p-type MOSFET Q12, and a second battery 52 is connected to the drain of the sixth p-type MOSFET Q14. Each MOSFET is controlled by a single control line (Q9-/B1C, Q10-/B1S, Q11-/B1L, Q12-/B2L, Q13-/B2C, Q14-/B2S).
When power from the DC supply 54 is available, the DC supply 54 powers the load 56 and the charger 42. The charger 42 can provide charge to either the first battery 50 or the second battery 52 by selecting the first switching cell 44 or the third switching cell 48. Charging of the first battery 50 is accomplished by selecting the first MOSFET Q9 and the second MOSFET Q10 via the control lines /B1C and /B1S, while charging of the second battery 52 is accomplished by selecting the fifth MOSFET Q13 and the sixth MOSFET Q14 via the control lines /B2C and /B2S. When power from the DC supply 54 is not available, the first battery 50 can provide power to the load 56 by selecting the second p-type MOSFET Q10 of the first switching cell 44 via control line /B1S and the third p-type MOSFET Q11 of the second switching cell 46 via control line /B1L. Alternatively, the second battery 52 can provide power to the load 56 by selecting the sixth p-type MOSFET Q14 of the third switching cell 48 via control line /B2S and the fourth p-type MOSFET Q12 of the second switching cell 46 via control line /B2L.
In some applications it is desirable to add additional batteries to the multiple battery systems. Four additional switches and two control lines are required for each additional battery added to the configuration of FIG. 1, while three switches and three control lines are required for each additional battery added to the configuration of FIG. 2. It is desirable to provide a configuration that reduces the number of parts in a multiple battery system to reduce costs and complexity, in addition to providing a configuration that facilitates the expansion of additional batteries in a multiple battery system with reduced parts.